I believe Sketch would do well on the iPad. Their success would be driven by their reputation and loyal user base from the Mac, not because the iOS App Store is a platform conducive to professional apps. Pixelmator took their Mac app to iPad and iPhone and now do really well on iOS. Vendors like Microsoft Office and Adobe can comfortably bring their apps to the iPad because they know their user base will carry them. The root problems of the App Store affect new apps made by new developers, not the incumbents.
Sketch may face longer term revenue growth from iOS due to the (lack of) upgrade model but I think a decent 1.0 iPad Sketch app would recoup its investments costs easily, making it a low risk bet.